Summary On 15 February 2001, C-FBUS, Skyservice Flight 6315, an AirbusA330, departed from Medan, Indonesia, at 1055 Coordinated Universal Time (1655 local time) on an approved extended range twin-engine operations (ETOPS) charter flight to Jeddah, Saudi Arabia. On board were 355passengers and 11crew members. About 3hours 8minutes after departure, while the aircraft was climbing from flight level (FL)350 to FL390, a loud bang was heard and the No.2engine (Pratt WhitneyPW4168, serial numberP733335) failed. The engine was shut down as per the electronic centralized aircraft monitoring (ECAM) checklist: the fire button was pushed and one fire bottle was discharged. The aircraft was approximately 30nautical miles west of Calicut, India, at latitude 11N, longitude 76E. The captain initiated a descent to FL230 and began a diversion to Colombo, SriLanka. He then briefed the cabin service manager, who in turn briefed the flight attendants and the passengers. The aircraft was vectored for a single-engine instrument landing system (ILS) approach to Runway22. Ninety minutes after the engine failure, the flight crew conducted an uneventful single-engine overweight landing with emergency response services standing by. This incident was the second engine failure on this aircraft. The first was the failure of the No.1engine on 05February2001, 13engine cycles before this incident. The first engine failure was investigated by Indonesia, with a TSB investigator as an accredited representative. Both engines were sent to Pratt Whitney facilities in Cheshire, Connecticut, where they were dismantled and examined in the presence of TSB investigation staff. All of the information from the engine examinations was sent to Indonesia. Ce rapport est galement disponible en franais. Other Factual Information The No.1 engine (Pratt and WhitneyPW4168, serial numberP733336) of this aircraft failed on 05February2001. The aircraft had been operating as Flight6308, a ferry flight from Jeddah, Saudi Arabia, to Solo, Indonesia. One to two hours into the flight, the crew received a No.1engine electronic centralized aircraft monitoring (ECAM) N2vibration advisory. At the time, the crew did not detect any associated airframe vibration. The engine vibration was analyzed by the aircraft maintenance engineer who was on board for the flight and by the maintenance chief at Solo, Indonesia, who was communicating via radio. They concluded that there was likely some damage to the engine fan blades and that the damage would be assessed and repaired when the aircraft reached the maintenance base at Solo. Based on this information, the captain elected to continue the flight. Approximately 4.5hours later, the flight crew noted the smell of engine odours, followed almost immediately by the onset of an airframe vibration. The ECAM warning message Eng1STALL displayed and, as the flight crew began the engine shutdown procedure, the ECAM message Eng1FAIL displayed. The engine was shut down as per the ECAM checklist: the fire button was pushed and one fire bottle was discharged. The flight crew then declared an emergency with Colombo Radio, decelerated the aircraft to green dot (single-engine) speed, and descended to flight level (FL)250. They proceeded to Medan, Indonesia, at reduced speed with continuous N1vibration and sporadic airframe vibration from the windmilling engine. Upon reaching Medan, the crew flew radar vectors for a single-engine approach to Runway05. The aircraft landed uneventfully after 2hours 20minutes of single-engine operation. The No.1 engine was removed and replaced, and the aircraft was returned to service on 10February2001. The engine was being prepared for shipment to the Pratt Whitney facilities at Cheshire, Connecticut, when the No.2 engine failed on 15February2001. Engine Information Failure of No. 1 engine (left side) on 05 February 2001: Part number: PW4168 Serial number: P733336 Date of manufacture: 28 March 1995 Time since new: 18350.54 hours Cycles since new: 5121 Date of last shop visit: (overhaul) March 1999 Time since shop visit: 6906.20 hours Cycles since shop visit: 1306 When this engine was disassembled for overhaul in March1999, some of the second-stage turbine blades were found to be corroded in the under-platform region, and the entire set of blades was replaced. The replacement blades had been refurbished and were from two different sources. These blades had been coated with PWA545 corrosion protection coating before installation. Failure of No.2 engine (rightside) on 15February2001: Part number: PW4168 Serial number: P733335 Date of Manufacture: 27 September 1994 Time since new: 19906.25 hours Cycles since new: 5257 Date of last shop visit: (overhaul) July 1999 Time since shop visit: 5199.11 hoursCycles since shop visit: 1020 When this engine was disassembled for overhaul in July1999, some of the second-stage turbine blades were found to be corroded in the under-platform region, and the entire set of blades was replaced. The replacement blades were new blades, coated with PWA36395-1 (Platinum Aluminide) corrosion protection coating. After the two engine failures, both engines were returned to the Pratt Whitney facilities at Cheshire, Connecticut, where they were dismantled and examined. In both cases, the engine failure was determined to be the result of a stress corrosion fracture of a second-stage turbine blade. History of Stress Corrosion The first stress corrosion failure of a second-stage turbine blade in a Pratt Whitney PW4000-series engine occurred in June1995. Since then, several service bulletins have addressed the stress corrosion issue in the Pratt Whitney2000-, Pratt Whitney4000-, and International Aero Engines V2500-series engines. This problem appears to affect some operators but not others. Skyservice Airlines Inc. is one of the operators whose engines were susceptible to stress corrosion cracking. To address the problem of premature engine failures due to stress corrosion cracking of the turbine blades, Pratt Whitney has been applying sacrificial corrosion protection coatings to the under-platform area of the second-stage turbine blades. In June 1996, Pratt Whitney began using the corrosion protection coating PW545 on the second-stage turbine blades. In February1997, Pratt Whitney issued Alert Service BulletinPW4G-100-A72-88, an instruction to collect and analyze dirt contaminants from the under-platform cavity of the second-stage turbine blades to determine the salt content of the dirt. The salt content was used as a measure of the corrosive environment inside the engine. In June1999, a newer coating (PWA36395-1, known as Platinum Aluminide) was introduced. This coating had been shown to provide twice the corrosion protection of the PWA545 coating. In April2000, Pratt Whitney changed the metallurgical composition of the second-stage turbine blades from a PWA1484 alloy to a PWA1480alloy. The PWA1480alloy is known to be more corrosion resistant. In May2000, a third type of corrosion protection coating (PWA36330) for the PWA1484blades was introduced. This coating was shown to be five times more effective than the Platinum Aluminide coating. Pratt Whitney stated that the failure of engineP733336 on 05February2001, with only 1306cycles, was within statistically predictable time limits; however, the failure of engineP733335 with 1020cycles on 15February2001 was premature and unexpected. During the examination of the second-stage turbine blades of both engines, it was noted that the corrosion on the uncoated portion of the blade roots was significantly more severe on engineP733335, even though the blades had less time in service. These corrosion pits averaged 0.012inch in depth, whereas on engineP733336 the depth of the corrosion pits averaged only 0.006inch. This finding was particularly notable because the blades from engineP733336 had previous time in service and might have had some small degree of pitting before installation in this engine, but the blades from engineP733335 were new at installation. As part of the investigation into the corrosive environment of the under-platform cavity of the second-stage turbine blades, Pratt Whitney recovered and analyzed dirt samples from both engines. Calcium magnesium carbonate (dolomite) and calcium sulfate (anhydrite) were identified as significant constituents of this dirt. Since Skyservice Airlines Inc. is based at Toronto, Ontario, Pratt Whitney also recovered dirt samples from the Toronto/LesterB.Pearson International Airport. Dolomite was a major constituent of these dirt samples. Dolomite is a very common mineral, occurring in a variety of geologic settings, including the Toronto area of southern Ontario; however, calcium sulfate (anhydrite) is not normally found in the Toronto area. During normal engine operation, environmental contaminants are ingested and carried throughout the engine along the internal airflow paths. The engine manufacturer uses internal air seals to maintain airflow where it is desirable and to inhibit airflow where it is not. One area where airflow is undesirable is the under-platform cavity of the second-stage turbine blades. The accumulation of dirt in this area is an indication that airflow was present. A visual examination of the rear face of the turbine blades, conducted to determine the status of the rear side plate seals, showed a discrepancy between the two engines. Witness marks on the aft face of the second-stage turbine blades of engine P733336 indicated that the rear side plate seal had been in place before the engine failure. EngineP733335 had no such witness marks, indicating that the rear side plate seal was not contacting the rear face of the turbine blades, thereby providing a path for airflow out of the under-platform cavity. Both rear side plate seals had been repaired during the respective overhauls. After this finding, other engines were examined to determine the status of the rear side plate seals. In five engines that showed no signs of corrosion attacks, the rear side plate seal contact was described as either excellent or good. In eight engines where corrosion was evident, the seal contact was described as either moderate or poor. Engine P733336 was assessed as moderate, engineP733335 as poor. The result of leaking rear side plate seals is that hot gas path air is allowed into the under-platform cavity and allows sulfur, a residue of jet fuel combustion, to mix with the calcium from the dolomite to form calcium sulfate (anhydrite). Auto Thrust In both events, after the engine failure, the flight crews found that the auto throttles could not be used. Although this did not cause either flight crew a significant problem, it did add slightly to their workload and was contrary to their training expectations. Auto thrust engagement is controlled by the flight management, guidance, and envelope computer (FMGEC), which receives relevant signals from each engine full authority digital engine control (FADEC). If the FMGEC detects an engine-out situation, it will authorize the auto thrust on the operating engine. When the engine is shut down using the engine fire pushbutton, the electronic engine control/ electrical control unit (EEC/ECU) reverts to an N1control mode from the engine pressure ratio mode. If the engine is still windmilling at speeds above a threshold of 3.8%-4.8%N2 while in the N1 control mode, the FMGEC will detect the electrical signal from the permanent magnet alternator (PMA) and will interpret this signal to mean that the engine is still operating. Since the FMGEC software logic has determined that both engines are operating, but that one is not responding, it will not authorize the auto thrust. Airbus is currently reviewing this software logic.